THE ROLE OF STOCHASTICITY IN SAWTOOTH OSCILLATIONS

Stochastization of field lines, resulting from the interaction of the fundamental m/n = 1/1 helical mode with other periodicities, plays an important role in sawtooth oscillations. The time-scale for stochastic temperature diffusion is shown to be sufficiently fast to account for the fast sawtooth crash. The enhanced electron and ion viscosities arising from the stochastic field lines are calculated. The enhanced electron viscosity always leads to an initial increase in the growth rate of the mode - the 'magnetic trigger'. The enhanced ion viscosity can ultimately lead to mode stabilization before a complete temperature redistribution or flux reconnection has occurred. A dynamical model is introduced to calculate the path of the sawtooth oscillation through the parameter space of shear and amplitude of the helical perturbation, including a stochastic trigger to an enhanced growth rate, stabilization by ion viscosity and a prescription for flux reconnection at the end of the growth phase. The model predicts that four types of sawtooth oscillation are possible, even for a plasma in a monotonic q(r) profile. There are two types of rearrangement of the magnetic configuration: a partial magnetic reconnection for which the value of the safety factor on axis q(0) oscillates around 0.7 with delta-q(0) congruent-to 0.05, and a full magnetic reconnection for which q(0) oscillates between 1 and about 0.8. In a partial reconnection, either the temperature flattening is rapid and is limited to an annulus near the q = 1 rational surface, which then gradually propagates to the axis, or the temperature on axis T(e)(0) decays rapidly when the stochastic region invades the geometrical axis. In a full magnetic reconnection, T(e)(0) collapses either rapidly when stochasticity is present or slowly without stochasticity. The main features of the model are compared with experimental observations. In particular, it is found that they can explain the sudden growth of the helical perturbation, the 'magnetic trigger', the fast time-scale of the temperature collapse, the partial temperature collapse and the persistence of an m/n = 1/1 island throughout the sawtooth cycle.